Boiler Water Cycle of a Fluidized Bed Reactor and a Fluidized Bed Reactor

ABSTRACT

A boiler water cycle of a fluidized bed boiler, preferably, a supercritical once-through-unit (OTU) boiler, includes a drop leg, a number of horizontal inlet headers, substantially having a length of the length of a front wall of the furnace of the boiler, arranged below the furnace, and panels of water tubes of the front wall and a rear wall, extensions of the water tubes being directly connected to the inlet headers, wherein each inlet header is in flow communication with a drop leg by way of an inlet duct connected to the end of the inlet header. The inlet headers include a front wall chamber below the front wall of the furnace, a rear wall chamber arranged below the rear wall of the furnace, and at least one grid chamber below the center part of the furnace grid in a wind box of the fluidized bed boiler.

This application is a U.S. national stage application of PCT International Application No. PCT/F12007/050285, filed May 21, 2007, and published as PCT Publication No. WO 2007/135240 A2, and which claims priority from Finnish patent application number 20060494, filed May 19, 2006.

FIELD OF THE INVENTION

The present invention relates to a boiler water cycle of a fluidized bed boiler (FB boiler) and to a fluidized bed boiler having such a boiler water cycle. The invention especially relates to a boiler water cycle of a 400 MW_(e) supercritical circulating fluidized bed boiler (CFB) operating on a once-through principle.

BACKGROUND OF THE INVENTION

In FB boilers, as in other thermal power boilers, evaporation, i.e., boiling, of the preheated inlet water, takes place mainly by means of water tube panels in the outer walls of a boiler furnace. The water to be evaporated is mostly led either from the steam drum of a drum boiler, or from the preheating surfaces for water in a once-through utility boiler, to the lower part of the boiler by one or more drop legs. The drop leg is usually connected to a number of inlet ducts, by means of which, water is introduced to inlet headers arranged below the furnace, which inlet headers have a length corresponding to the width of the furnace walls. Water tubes in the water tube panels of the outer walls of the furnace, in turn, are connected to the inlet headers to heat up and to evaporate water in the water tubes. The water tubes of the outer walls are connected from the upper end to outlet headers and pipings, by means of which, steam is led further to water separation and super heating.

In order to be able to guarantee a uniform distribution of water in the water tubes of the water tube panel, the drop leg is usually connected to a large number of inlet ducts, which are connected from one end approximately at equal intervals to the whole length of the inlet headers. Once-through utility boilers, which have such a large number of inlet ducts, are disclosed, for example, in U.S. Pat. No. 4,290,389, No. 3,399,656 and No. 3,369,526. U.S. Pat. No. 4,183,330 discloses an example of an FB boiler, having a number of inlet lines connecting drop legs of a steam drum to an annular inlet header, which introduces water to the wall tubes of the furnace.

The drop leg may be substantially vertical, whereby it generally ends at the outside of the bottom level of the boiler, or the lower part thereof may be turned to horizontal, then being able to extend as such below one of the boiler walls. In the latter case, the inlet ducts of the inlet header, being connected to the wall, may be relatively short. Especially, when there are two drop legs, they may preferably extend below the longer sidewalls, in other words, the front wall and the rear wall, or, alternatively, below the shorter sidewalls thereof.

The boiler water cycles described above are working solutions as such, but in large boilers, they may become rather complicated. The boiler water cycle becomes especially complicated when the bottom grid of the furnace is also cooled by evaporation tubes and, due to the large size of the grid, it is advantageous, and in drum boilers, even necessary, to locate one or more inlet headers to run in the longitudinal direction below the center part of the grid. Especially, with fluidized bed boilers, the arrangement of inlet ducts of a so-called grid chamber is problematic, because, in addition to an inlet chamber for fluidizing air, a so-called wind box must be arranged in the fluidized bed boiler below the grid. If the wind box is desired to be arranged as one large, undivided construction, which is advantageous in view of homogeneous air distribution, the grid chamber should generally be located inside the wind box. Thereby, the numerous inlet ducts must be led through the wind box.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a boiler water cycle of a fluidized bed boiler, which diminishes problems related to boiler water cycles of a fluidized bed boiler in accordance with the prior art.

Especially, an object of the invention is to provide a simple and reliable boiler water cycle of a supercritical circulating fluidized bed boiler operating on a once-through principle.

A further object of the invention is to provide a fluidized bed boiler having such a boiler water cycle.

In order to solve the above-mentioned prior art problems, a boiler water cycle of a fluidized bed boiler and a fluidized bed boiler having such a boiler water cycle are provided, the characteristic features of which are recited in the independent apparatus claims.

Thus, a boiler water cycle of a fluidized bed boiler in accordance with the present invention comprises a drop leg and a number of horizontal inlet headers, substantially of the length of a front wall of the boiler furnace, arranged below the furnace of the fluidized bed boiler, panels of water tubes of the front wall and of a rear wall, the extensions of the water tubes being directly connected to the inlet headers, and each inlet header being in flow communication with the drop leg merely by means of an inlet duct connected to the end of the inlet header.

The furnace of the fluidized bed boiler is usually of a horizontal cross section rectangular design, and the front wall and the rear wall of the furnace usually refer to longer walls of the furnace. The shorter side walls of the furnace may preferably also be cooled in accordance with the present invention, but it is possible that the feed of water to the shorter walls of the furnace is carried out in a conventional way by utilizing a number of inlet ducts. A third alternative that comes into question, especially when the lengths of the longer or shorter walls of the furnace are relatively close to each other, is that the shorter walls of the furnace are cooled in accordance with the invention, and the longer walls are coated in a conventional manner.

When each inlet header is in flow communication with the drop leg, according to the present invention, preferably, only with one drop leg, merely by means of an inlet duct connected to the end of the inlet header, the complexity caused by numerous inlet ducts is avoided. Connecting to the end of the inlet header refers, in this connection, either to an inlet duct being connected parallel to the inlet header right to the end thereof or the inlet duct being connected to the side wall of the inlet header, but substantially, to the first end thereof. The arrangement in accordance with the invention is especially advantageous in large circulating fluidized bed boilers, in which it is desired to form an undivided wind box enabling a homogeneous flow of fluidizing gas. The manufacture of such is severely hampered by the numerous inlet lines required by the prior art.

Naturally, a disadvantage of the arrangement in accordance with the present invention is that the inner diameters of the inlet headers must be large enough to be able to ensure sufficient boiler water flow also to the far end of the inlet header. The required size of the inlet headers depends, thus, on the amount of water to be fed, but, according to a preferred embodiment, the inner diameter of the inlet headers is at least 200 mm, most preferably, at least 300 mm. Large inlet headers as such increase costs, but the inventor of the present invention has surprisingly noticed that with large FB boilers, especially, with supercritical once-through CFB boilers, the power output of which is at least 400 MW_(e), it is advantageous to use the above-described, very simple arrangement for inlet headers for the boiler water.

Especially, when the boiler water cycle in accordance with the invention is a supercritical once-through cycle, an especially simple and advantageous arrangement is provided, when there is only one drop leg, whereby, each of the inlet headers is in flow communication with one common drop leg.

According to a preferred embodiment of the present invention, the inlet headers comprise a front wall chamber arranged below the front wall of the furnace, a rear wall chamber arranged below the rear wall of the furnace and at least one, so-called grid chamber below the center part of the furnace grid. In this preferred embodiment, generally, a first portion of the extensions of the water tubes in the front wall of the furnace is connected directly to the front wall chamber and, correspondingly, a first portion of the extensions of the water tubes in the rear wall of the furnace is connected directly to the rear wall chamber. According to the arrangement, not all of the water tubes of the water tube panel of the front wall and the rear wall are connected to the above-mentioned front wall chamber and the rear wall chamber, but a second portion of the water tubes of the front wall and of the rear wall extends as grid tubes parallel to the furnace grid to the grid chamber. By utilizing this arrangement, it is possible to provide a uniform distribution of the boiler water to all grid tubes, too. The grid chambers are preferably arranged below the furnace grid, inside the wind box.

Since the strength requirements of the grid tubes are higher than the requirements of the water tubes of the front walls and the rear walls, and since enough space must remain between the grid tubes for the nozzles for fluidizing air, the grid tubes are usually of a larger diameter than the water tubes of the walls. Therefore, each grid tube is preferably connected by means of a special fitting member to a water tube of the above-mentioned second portion of the water tubes in the front wall or the rear wall.

It is advantageous in large boilers to have two grid chambers, whereby the second portion of the extensions of the water tubes in the front wall are preferably connected to the first grid chamber and the second portion of the extensions of the water tubes in the rear wall are connected to the second grid chamber. The water tubes of the first and second portions preferably alternate in the front wall and the rear wall, whereby, for example, every second water tube of the front wall is in connection with the front wall chamber, and the rest of them are in connection with the first grid chamber.

A significant additional advantage of the large inlet headers is that they can be arranged as support structures of the lower part of the furnace, whereby they decrease the number of other supporting structures. Especially, in large FB boilers, it is possible to simplify the support of the center part of the grid, when a large grid chamber in accordance with a preferred embodiment of the present invention forms a part thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is discussed more in detail with reference to the accompanying drawings, in which

FIG. 1 schematically illustrates a side view of a circulating fluidized bed boiler, comprising a boiler water cycle in accordance with a preferred embodiment of the present invention;

FIG. 2 schematically illustrates a vertical sectional view of a lower part of a circulating fluidized bed boiler, comprising a boiler water cycle in accordance with a preferred embodiment of the present invention; and

FIG. 3 schematically illustrates a detail of a lower part of boiler water tubes of a circulating fluidized bed boiler in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a CFB boiler 10, in accordance with a preferred embodiment of the present invention, comprising a furnace 12. The boiler 10 in accordance with the invention may be a natural circulation boiler, in other words, a drum boiler, but it is, most preferably, a supercritical once-through utility boiler, which is illustrated, for example, in FIG. 1. The horizontal cross section of the furnace is usually rectangular, and it is limited by a bottom, a ceiling and sidewalls, of which one long sidewall, a so-called front wall 14, is shown in FIG. 1. The walls limiting the furnace are conventionally manufactured as a water tube wall construction, in other words, from water tubes 16 and fins connected gas tightly therebetween. The water tubes and fins form water tube panels 18, which are used for boiling water, i.e., for converting preheated feed water to steam.

A so-called wind box 20 is arranged below the furnace for supplying primary gas, generally, air, required for the combustion of fuel and for the fluidization of the fluidized bed, to the furnace. Other conventional parts of the CFB boiler 10, such as fuel inlet means, discharge channels for flue gases and bottom ash, as well as particle separators and return ducts related thereto, are also connected to the furnace 12. For simplicity, these details, which are irrelevant to the present invention, are not shown in FIG. 1.

The preheated feed water 22 led from the water preheating surfaces, so-called economizers, and the possible liquid returned from the steam separator 24 are led by means of a drop leg 26 to the level of the furnace bottom, from where it is distributed by means of inlet ducts 28 to the inlet headers 30 of the evaporator tubes in the sidewalls of the boiler 10. According to the conventional technique, multiple inlet ducts are connected, approximately equally spaced throughout the whole length of the inlet headers 30. It is, however, characteristic of the present invention that each inlet header 30 is in flow connection with a drop leg 26 merely by means of an inlet duct 28 connected to the end of the inlet header 30. To enable this, the diameter of the inlet headers 30 must naturally be sufficient, and substantially greater than that in the prior art arrangement. The inner diameter of the inlet headers 30 in accordance with the invention is preferably at least 200 mm, most preferably, at least 300 mm. The structure of the inlet pipings in accordance with the invention is very simple, and it does not disturb the location of the apparatuses connected to the lower part of the furnace 12, nor, for example, the formation of an extensive, undivided wind box 20.

The water from inlet headers 30 is led to the water tube panels 18 to evaporate and further, as steam to the outlet headers 32. If the boiler 10 is a so-called drum boiler, the force driving water and steam upwards in the panels is the weight of the liquid column in the drop leg of the drum. If, in turn, the boiler 10 is a so-called forced circulation boiler, especially, a so-called supercritical once-through boiler, the driving force is the pressure generated by the pump of the water cycle (not shown in FIG. 1). The steam from the outlet headers 32, possibly still containing some liquid water, is led to the water and steam separating apparatus 24 by means of collector tubes 34. The steam continues further in the steam pipings 36 to the superheaters arranged, for example, in the flue gas channel.

FIG. 2 schematically illustrates a simplified vertical cross section of a lower part of the furnace 12 of a fluidized bed boiler 10 having a water cycle in accordance with a preferred embodiment of the present invention. FIG. 2 shows a front wall 14 and a rear wall 38, which are formed of water tube panels of the furnace 12, as well as a wind box 20. FIG. 2 also schematically illustrates the wind box 20 with fluidizing gas nozzles 40, which are arranged between the grid tubes 42.

Extensions 44, 46 of the first portion of the water tube in the front wall 14 and the rear wall 38 are connected directly to a front wall chamber 48 and a rear wall chamber 50, respectively. The front wall chamber 48 and the rear wall chamber 50 are both connected in a manner shown in FIG. 1 to a drop leg merely by means of an inlet duct connected to the end of the chamber. Since, thereby, there are other inlet ducts connected to the inlet headers, in accordance with the present invention, each cross section of the furnace 12 is simple, in that there are no inlet ducts of the inlet headers hampering the connection of other apparatuses to the lower part of the furnace 12.

In the embodiment of FIG. 2, there are two other inlet headers arranged in the wind box 20, so-called first and second grid chambers 52, 54. Grid tubes 42 are connected to the grid chambers 52, 54, each of which is preferably connected to a water tube of the front wall 14 or the rear wall 38 in a manner discussed below. Since the grid chambers 52, 54, too, are connected in a manner illustrated in FIG. 1 to the drop leg merely be means of an inlet duct connected to the end of the chamber, there are no inlet ducts connected to the center part of the grid chambers 52, 54, which would hamper the formation of an undivided wind box. The grid chambers 52, 54 extending throughout the length of the boiler walls also significantly reinforce the grid structure and, thus, diminish the need for other supporting structures.

FIG. 3 schematically illustrates a detail of a lower part of the boiler water tubes in a circulating fluidized bed boiler in accordance with a preferred embodiment of the present invention. This drawing shows a front wall chamber 48, a first grid chamber 52 and the water tubes connected thereto. Naturally, the drawing could also illustrate, correspondingly, water tubes connected to a rear wall chamber and a second grid chamber. As it was shown earlier in connection with FIG. 2, the grid tubes are preferably arranged longitudinally at the center part of the grid cross section, and the length of the substantially horizontal portion of the grid tubes 42 parallel to the grid is, thus, approximately half of the whole width of the grid.

The grid tubes 42 connected to the first grid chamber 52 run from the grid chamber first, to a certain extent upwards and then, turn parallel to the grid towards the front wall 14, where they again turn upwards. Since the diameter of the grid tubes 42 is preferably greater than the diameter of the water tubes 54, 54′ of the furnace wall, the grid tubes 42 are preferably connected by fitting members 56 to the water tubes 54′ of the furnace wall. Advantageously, every second of the tubes of the furnace wall belongs to the so-called first portion 54 of the water tubes, the extensions 44 thereof being connected directly to the front wall chamber 48, and the rest of the tubes belong to a so-called second portion 54′, which is connected by means of fitting member 56 to the grid tubes 42 and, therethrough, to the first grid chamber 52.

The present invention has been described with reference to some exemplary arrangements. These arrangements have not been given to limit the scope of the invention, but the invention is solely limited by the patent claims and the definitions given therein. 

1. A boiler water cycle of a fluidized bed boiler, the boiler water cycle comprising a drop leg; a number of horizontal inlet headers, substantially having a length of the length of a front wall of a furnace of the boiler, arranged below the furnace, and panels of water tubes of the front wall and a rear wall, extensions of the water tubes being directly connected to the inlet headers, wherein each inlet header is in flow communication with a drop leg by an inlet duct connected to the end of the inlet header, wherein the inlet headers comprise a front wall chamber below the front wall of the furnace, a rear wall chamber arranged below the rear wall of the furnace, and at least one grid chamber below the center part of the furnace grid in a wind box of the fluidized bed boiler.
 2. A boiler water cycle according to claim 1, wherein the boiler water cycle is a supercritical once-through cycle.
 3. A boiler water cycle according to claim 1, wherein each inlet header is in flow communication with only one drop leg.
 4. A boiler water cycle according to claim 3, wherein each inlet header is in flow communication with a common drop leg.
 5. A boiler water cycle according to claim 1, wherein a portion of the extensions of water tubes in the front wall are connected directly to the front wall chamber, a portion of the extensions of water tubes in the rear wall are connected directly to the rear wall chamber and another portion of the water tubes in the front wall and another portion of the water tubes in the rear wall extend parallel to the furnace grid as grid tubes connected to the grid chamber.
 6. A boiler water cycle according to claim 1, wherein the diameter of the water tubes in the front wall and rear wall is smaller than the diameter of the grid tubes, and each water tube of the other portion of the water tubes of the front wall and rear wall is connected to the grid tube by a fitting member.
 7. A boiler water cycle according to claim 5, wherein the inlet headers comprise two grid chambers and the other portion of the extensions of the water tubes in the front wall is connected to the first grid chamber and the other portion of the extensions of the water tubes in the rear wall is connected to the second grid chamber.
 8. A boiler water cycle according to claim 1, wherein the inner diameter of the inlet headers is at least 200 mm.
 9. A boiler water cycle according to claim 7, wherein the inner diameter of the inlet headers is at least 300 mm.
 10. A boiler water cycle according to claim 7, wherein the grid chamber is arranged to act as a supporting element of the grid.
 11. A fluidized bed boiler comprising: a boiler water cycle, the boiler water cycle comprising: (i) a drop leg; and (ii) a number of horizontal inlet headers, substantially of the length of a front wall of a furnace of the boiler, arranged below the furnace, and panels of water tubes of the front wall and a rear wall, extensions of the water tubes being directly connected to the inlet headers, wherein each inlet header is in flow communication with a drop leg merely by an inlet duct connected to the end of the inlet header, wherein the inlet headers comprise a front wall chamber below the front wall of the furnace, a rear wall chamber arranged below the rear wall of the furnace, and at least one grid chamber below the center part of the furnace grid in a wind box of the fluidized bed boiler.
 12. A bed boiler water cycle according to claim 2, wherein each inlet header is in flow communication with only one drop leg.
 13. A fluidized bed boiler according to claim 11, wherein the boiler water cycle is a supercritical once-through cycle.
 14. A fluidized bed boiler according to claim 11, wherein each inlet header is in flow communication with only one drop leg.
 15. A fluidized bed boiler according to claim 14, wherein each inlet header is in flow communication with a common drop leg.
 16. A fluidized bed boiler according to claim 11, wherein a portion of the extensions of water tubes in the front wall are connected directly to the front wall chamber, a portion of the extensions of water tubes in the rear wall are connected directly to the rear wall chamber and another portion of the water tubes in the front wall and another portion of the water tubes in the rear wall extend parallel to the furnace grid as grid tubes connected to the grid chamber.
 17. A fluidized bed boiler according to claim 11, wherein the diameter of the water tubes in the front wall and rear wall is smaller than the diameter of the grid tubes, and each water tube of the other portion of the water tubes of the front wall and rear wall is connected to the grid tube by a fitting member.
 18. A fluidized bed boiler according to claim 16, wherein the inlet headers comprise two grid chambers and the other portion of the extensions of the water tubes in the front wall is connected to the first grid chamber and the other portion of the extensions of the water tubes in the rear wall is connected to the second grid chamber.
 19. A fluidized bed boiler according to claim 11, wherein the inner diameter of the inlet headers is at least 200 mm.
 20. A fluidized bed boiler according to claim 18, wherein the inner diameter of the inlet headers is at least 300 mm.
 21. A fluidized bed boiler according to claim 18, wherein the grid chamber is arranged to act as a supporting element of the grid.
 22. A fluidized bed boiler according to claim 12, wherein each inlet header is in flow communication with only one drop leg. 